Discovery
Velocity of Proxima Centauri towards and away from the Earth as measured with the HARPS spectrograph during the first three months of 2016. The red symbols with black error bars represent data points, and the blue curve is a fit of the data. The amplitude and period of the motion were used to estimate the planet's minimum mass.
Proxima Centauri had become a target for exoplanet searches already before the discovery of Proxima Centauri b, but initial studies in 2008 and 2009 ruled out the existence of larger-than-Earth exoplanets in the habitable zone.[4] Planets are very common around dwarf stars, with on average 1-2 planets per star,[5] and about 20-40% of all red dwarfs have one in the habitable zone.[6] Additionally, red dwarfs are by far the most common types of stars.[7]
Before 2016, observations with instruments[a] at the European Southern Observatory in Chile had identified anomalies in Proxima Centauri[8] which could not be satisfactorily be explained by flares[b] or chromospheric[c] activity of the star. Anglada-Escudé et al. 2016 proposed that an exoplanet in the habitable zone of Proxima Centauri could explain these anomalies.[11] In 2020, another planet Proxima Centauri c was discovered,[12] while the existence of a dust belt around Proxima Centauri and of a third planet were as of 2021 unconfirmed.[13] The discovery of Proxima Centauri b, a planet at habitable distances from the closest star to the Solar System, was a major discovery in planetology[14] and has drawn interest to the Alpha Centauri star system that Proxima is a member of.[15]
Physical properties
Proxima Centauri b is the closest exoplanet to Earth,[16] being at a distance of about 4.2 ly.[3] It orbits Proxima Centauri every 11.18427±0.00070 Earth days at a distance of 0.0485 au, [14] over 20 times closer to Proxima Centauri than Earth is to the Sun.[17] As of 2021 it is unclear if it actually has an eccentricity[d][20] but Proxima Centauri b is unlikely to have any obliquity.[21] The age of the planet is unknown;[22] Proxima Centauri itself may have been captured by Alpha Centauri and thus not necessarily of the same age as the latter, which are about 5 billion years old.[13] Proxima Centauri b is unlikely to have stable orbits for moons.[23]
As of 2020, the estimated minimum mass of Proxima Centauri b is 1.173±0.086 M🜨;[14] other estimates are similar[24] but all estimates are dependent on the inclination of the planet's orbit and may be underestimates.[13] This makes it similar to Earth, but the radius of the planet is poorly known and hard to determine[25] and the mass borders on the cutoff between Earth-type and Neptune-type planets.[5] Depending on the composition, Proxima Centauri b could either be a Mercury-like planet with a large core—which would require particular conditions early in the planet's history—to a very water-rich planet. Observations of the Fe-Si-Mg ratios of Proxima Centauri may allow a determination of the composition of the planet[26] since they are expected to roughly match these of the planets; various observations have found Solar System-like ratios of these elements.[27]
Relatively little is known about Proxima Centauri b as of 2021—mainly its distance from the star and its orbital period[28]—but a number of simulations of its properties have been made.[13] A number of simulations and models have been created that assume Earth-like compositions[29] and include predictions of the galactic environment, internal heat generation from radioactive decay and magnetic induction heating[e], planetary rotation, the effects of stellar radiation, the amount of volatile species the planet consists of and the changes of these parameters over time.[27]
Proxima Centauri b likely developed under different conditions than Earth, with less water, stronger impacts and an overall faster development assuming that it formed at its current distance from the star.[31] Proxima Centauri b probably did not form at its current distance to Proxima Centauri, as the amount of material in the protoplanetary disk would be insufficient. Instead, it or fragments formed at larger distances and then migrated to the current orbit of Proxima Centauri b. Depending on the nature of the precursor material, it may be rich in volatiles.[11] A number of different formation scenarios are possible, many of which depend on the existence of other planets around Proxima Centauri and which would result in different compositions.[32]
Tidal locking
Proxima Centauri b is likely to be tidally locked to the host star,[23] which for an 1:1 orbit would mean that the same side of the planet would always face Proxima Centauri.[22] It is unclear if habitable conditions can arise under such circumstances[33] as an 1:1 tidal lock would lead to an extreme climate with only part of the planet habitable.[22]
However, the planet may not be tidally locked. If the eccentricity of Proxima Centauri b was higher than 0.1[34]-0.06, it would tend to enter a Mercury-like 3:2 resonance[f] or higher-order resonances such as 2:1.[35] Additional planets around Proxima Centauri and interactions[g] with Alpha Centauri could excite higher eccentricies.[36] If the planet isn't symmetrical (triaxial), a capture into a non-tidally locked orbit would be possible even with low eccentricity.[37] A non-locked orbit however would result in tidal heating of the planet's mantle, increasing volcanic activity and potentially shutting down a magnetic field-generating dynamo.[38] The exact dynamics are strongly dependent on the internal structure of the planet and its evolution in response to tidal heating.[39]
Star
Main article: Proxima Centauri
Proxima Centauri is a red dwarf[35] with a mass equivalent to 0.120±0.015 solar masses and a radius of 0.141±0.021 solar radii. With an effective temperature[h] of 3050±100 kelvin, it has a spectral type[i] of M5.5V and a luminosity 0.00155±0.00006 of the Sun.[11] Proxima Centauri is a flare star and its luminosity varies by a factor of 100 over a timespan of hours.[42] The magnetic field of Proxima Centauri is considerably stronger than that of the Sun, with an intensity of 600±150 Gauss;[1] it varies in a 7-year long cycle.[43]
It is the closest star to the Sun[j], with a distance of 4.2426 ± 0.0020 light-years (1.3008 ± 0.0006 pc). Proxima Centauri is part of a multiple star system, whose other members are Alpha Centauri A and Alpha Centauri B which form a binary star subsystem.[44] The dynamics of the multiple star system could have caused Proxima Centauri b to move closer to its host star over its history.[45] The detection of a planet around Alpha Centauri in 2012 is considered questionable.[44] Despite its proximity to Earth, Proxima Centauri is too faint to be visible to the naked eye[4] with the exception of an instance where a flare made it visible to the naked eye.[46]
Surface conditions
Artist's conception of the surface of Proxima Centauri b. The Alpha Centauri AB binary system can be seen in the background, to the upper right of Proxima.
Proxima Centauri b is located within the classical habitable zone of its star;[47] it receives about 65% of Earth's irradiation. Its equilibrium temperature is about 234+6
−14 K.[11] Various factors, such as the orbital properties of Proxima Centauri b, the spectrum of radiation emitted by Proxima Centauri[k] and the behaviour of clouds[l] and hazes influence the climate of an atmosphere-bearing Proxima Centauri b.[52]
There are two likely scenarios for an atmosphere of Proxima Centauri b, one rich in oxygen and/or carbon dioxide if large amounts of water were converted to oxygen during the early phases of Proxima Centauri. and the hydrogen lost. Another when the planet initially featured a hydrogen-rich atmosphere or originated farther away from Proxima Centauri;[53] this would have reduced the escape of water and allowed it to persist on the planet.[45] If an atmosphere exists, it is likely to contain oxygen-bearing compounds such as oxygen and carbon dioxide. Together with the star's magnetic activity, they would give rise to aurorae that could be observed from Earth[54] if the planet has a magnetic field.[55]
Climate models including general circulation models used for Earth climate[56] have been used to simulate the properties of Proxima Centauri b's atmosphere. Depending on its properties such as whether it's tidally locked, the amount of water and carbon dioxide a number of scenarios are possible: Planets partially or wholly covered with ice, planet-wide or small oceans or only dry land, combinations between these[57] or scenarios with one or two "eyeballs"[m][59] or lobster-shaped areas with liquid water.[60] Additional factors are the nature of convection,[61] the distribution of continents, which can sustain a carbonate-silicate cycle and thus stabilize the atmospheric carbon dioxide concentrations,[62] ocean heat transport which broadens the space for habitable climates, salinity variations that alter the properties of an ocean,[59] the rotational period of the planet which determines Rossby wave dynamics[63] and sea ice dynamics which could cause a global ocean to freeze over.[64]
Stability of an atmosphere
The stability of an atmosphere is a major issue for the habitability of Proxima Centauri b:[65]
Strong irradiation by UV radiation and X-rays from Proxima Centauri constitutes a challenge to habitability.[16] Proxima Centauri b receives about 10-60 times as much of this radiation as Earth[47] with a particular increase in the X-rays[66] and might have received even more in the past,[67] adding up to 7-16 times as much cumulative XUV radiation than Earth.[68] UV radiation and X-rays can effectively evaporate an atmosphere[17] since hydrogen readily absorbs the radiation and does not readily lose it again, thus warming until the speed of hydrogen atoms and molecules is sufficient to escape from the gravitational field of a planet.[69] They can remove water by splitting it into hydrogen and oxygen and heating the hydrogen in the planet's exosphere until it escapes. The hydrogen can drag other elements such as oxygen[70] and nitrogen away.[71] Nitrogen and carbon dioxide can escape on their own from an atmosphere but this process is unlikely to substantially reduce the nitrogen and carbon dioxide content of an Earth-like planet.[72]
Stellar winds and coronal mass ejections are an even bigger threat to an atmosphere.[17] The amount of stellar wind impacting Proxima Centauri b may amount to 4-80 times that impacting Earth.[68] The more intense UV and X-rays radiation could lift the planet's atmosphere to outside of the magnetic field, increasing the loss triggered by stellar wind and mass ejections.[73]
At Proxima Centauri b's distance from the star, the stellar wind is likely to be denser than around Earth by a factor of 10-1000 depending on the strength of Proxima Centauri's magnetic field.[74] As of 2018 it is unknown whether the planet has a magnetic field[16] and the upper atmosphere may have its own magnetic field.[73] Depending on the intensity of Proxima Centauri b's magnetic field, it can penetrate deep into the atmosphere of the planet and strip parts of it off,[75] with substantial variability over daily and annual timescales.[74]
If the planet is tidally locked to the star, the atmosphere can collapse on the night side.[76] This is particularly a risk for a carbon dioxide-dominated atmosphere although carbon dioxide glaciers could recycle.[77]
Unlike Sun-like stars, Proxima Centauri's habitable zone would have been farther away early in the system's existence[78] when the star was in its pre-main sequence[n] stage.[79] In the case of Proxima Centauri, assuming that the planet formed in its current orbit it could have spent up to 180 million years too close to its star for water to condense.[45] Proxima Centauri b may therefore have suffered a Runaway greenhouse effect, in which the planet's water would have evaporated into steam,[80] which would then have been split into hydrogen and oxygen by UV radiation. The hydrogen and thus any water would have subsequently been lost,[45] similar to what is believed to have happened to Venus.[81]
While the characteristics of impact events on Proxima Centauri b are currently entirely conjectural, they could destabilize the atmospheres[82] and boil off oceans.[12]
Even if Proxima Centauri b lost its original atmosphere, volcanic activity could rebuild it after some time. A second atmosphere would likely contain carbon dioxide,[33] which would form a more stable atmosphere than an Earth-like atmosphere would be.[27] In the case of Earth, the amount of water contained within the mantle might approach that of one Earth ocean.[38] Additionally, impacts of exocomets could resupply water to Proxima Centauri b, if they are present.[83]
Delivery of water to Proxima Centauri b
A number of mechanisms can deliver water to a developing planet; how much water Proxima Centauri b received is unknown.[31] Modelling by Ribas et al. 2016 indicates that Proxima Centauri b would have lost no more than one Earth ocean equivalent of water[16] but later research suggested that the amount of water lost could be considerably larger[84] and Airapetian et al. 2017 concluded that an atmosphere would be lost within ten million years.[85] The estimates are strongly dependent on the initial mass of the atmosphere, however, and are thus highly uncertain.[38]
Life
See also: Habitability of red dwarf systems
In the context of exoplanet research, "habitability" is usually defined as the possibility that liquid water exists on the surface of a planet.[53] As normally understood in the context of exoplanet life, liquid water on the surface and an atmosphere are prerequisites for habitability—any life limited to the sub-surface of a planet,[78] such as in a subsurface ocean like in Europa in the Solar System, would be difficult to detect from afar[79] although it may constitute a model for life in a cold ocean-covered Proxima Centauri b.[86]
The habitability of red dwarfs is a controversial subject,[22] with a number of considerations:
Both the activity of Proxima Centauri and tidal locking would hinder the establishment of these conditions.[11]
Unlike XUV radiation, UV radiation on Proxima Centauri b is redder (colder) and thus may interact less with organic compounds[87] and may produce less ozone.[88] Conversely, stellar activity could deplete an ozone layer sufficiently to increase UV radiation to dangerous levels.[38][89]
Depending on its eccentricity, it may partially lie outside of the habitable zone during part of its orbit.[22]
Oxygen[90] and/or carbon monoxide may built up in the atmosphere of Proxima Centauri b to toxic quantities.[91] High oxygen concentrations may however aid in the evolution of complex organisms.[90]
If oceans are present, the tides could alternately flooding and drying coastal landscapes, triggering chemical reactions conducive to the development of life,[92] favour the evolution of biological rhythms such as the day-night cycle which otherwise would not develop in a tidally locked planet without a day-night cycle,[93] mix oceans and supply and redistribute nutrients[94] and stimulate periodic expansions of marine organisms such as red tides on Earth.[95]
On the other hand, red dwarfs like Proxima Centauri have a lifespan much longer than the Sun, up to many times the estimated age of the Universe, and thus give life plenty of time to develop.[96] The radiation emitted by Proxima Centauri is ill-suited for oxygen-generating photosynthesis but sufficient for anoxygenic photosynthesis[97] although it is unclear how life depending on anoxygenic photosynthesis could be detected.[98] One study in 2017 estimated that the productivity of a Proxima Centauri b ecosystem based on photosynthesis may be about 20% that of Earth's.[99]
Observation and exploration
As of 2021, Proxima Centauri b has not yet been directly imaged, as its separation from Proxima Centauri is too small for that.[100] It is unlikely[o] to transit Proxima Centauri from Earth's perspective.[101] The star is monitored for the possible emission of technology-related radio signals by the Breakthrough Listen project which in April-May 2019 detected the BLC1 signal; later investigations however indicated it is probably of human origin.[102]
Future large ground-based telescopes and space-based observatories such as the James Webb Space Telescope and the Wide-Field Infrared Survey Telescope could directly observe Proxima Centauri b, given its proximity to Earth,[17] but disentangling the planet from its star would be difficult.[33] Possible traits observable from Earth are the reflection of starlight from an ocean,[103] the radiative patterns of atmospheric gases and hazes[104] and of atmospheric heat transport[p].[105] Efforts have been done to determine how Proxima Centauri b would look like to Earth if it has particular properties such as atmospheres of a particular composition.[28]
Even the fastest spacecrafts build by humans would take a long time to travel interstellar distances; Voyager 2 would take about 75,000 years to reach Proxima Centauri. Among the proposed technologies to reach Proxima Centauri b in human lifespans are solar sails that could reach speeds of 20% the speed of light; problems would be how to decelerate a probe when it arrives in the Proxima Centauri system and collisions of the high-speed probes with interstellar particles. Among the projects of travelling to Proxima Centauri b are the Breakthrough Starshot project, which aims to develop instruments and power systems that can reach Proxima Centauri in the 21st century.
